The invention relates to reaction resins formed from anionically polymerisable di- or tri- functional epoxy compounds which are useful for impregnating and/or casting large-volume components, especially for cryotechnology.
The use of plastics such as impregnating resins, especially in cryotechnology, poses a problem because of the insufficient mechanical strength and flexibility of the plastics within a temperature range from about 4 to 100 K. Impregnating resins for large-volume components, for instance, those which serve for impregnating superconducting magnet coils, must meet a number of requirements which until development of the present invention could not be completely met. These are:
low starting viscosity (about 20 to 100 mPa. sec) PA1 long use time (up to 50 hours at the impregnating temperature) PA1 high mechanical strength level PA1 good adhesion on alloy steels and nonferrous metals (NE-metals) PA1 temperature-cycle resistance PA1 high reaction shrinkage in the liquid phase and low reaction shrinkage in the gel phase, as well as PA1 optionally, high radiation resistance (up to 3.10.sup.6 kGy).
Some reaction resin molded materials which are useful for cryotechnology and have improved strength and flexibility are known. These are, for instance, addition products based upon bisphenol-A-epoxy resins, dodecyl-succinic acid anhydride and aliphatic tricarboxylic acids as well as those based upon a mixture of bisphenol-A-resin, aliphatic epoxy resin, diurethane prepolymers and 4,4'-methylene-bis(2chloroaniline). Such compounds, however, generate only limited improvements.
Partially-crystalline cross-linked polymers are also known as thermal and electrical insulating material for cryotechnology. These polymers have a melting enthalpy of 30 to 80 kJ:kg.sup.-1 and have sequences which consist of 2 to 14 methylene groups and are connected to each other by oxycarbonyl groups (European Patent Application No. A1-0 042 356: page 8, claims 1 to 4). Such polymers, which are used in the form of vessels and tubes or as a component of laminates and composite materials, are, for example, cross-linked products of crystalline aliphatic polyesters with a molecular weight between 1,000 and 10,000, which contain acid end groups, and cross-linking agents (hardening agents) with at least three epoxy groups. In these systems, the high viscosity and the short use time have a detrimental effect.
There has also been described an insulating system for magnets used in fusion and high energy research which is suitable for winding temperatures in the range of -270.degree. to +155.degree. C. and said to have high radiation resistance and excellent mechanical strength ("Brown Boveri Mitteilugen", vol. 65, (1978), pages 326 to 333). The impregnating resin of this insulation is a modified radiation-resistant epoxy resin of the bisphenol-A-type, an anhydride hardener and a latent accelerator. It is believed that large insulating thicknesses and large coils which require considerable volumes of resin can be impregnated. Also known are impregnating resins of epoxies and primary or secondary amines as hardeners.
It is found in practice, however, that with large-volume parts, the use time of additively hardening systems, i.e., of systems of epoxies and acid anhydrides and primary or secondary amines, respectively, is barely sufficient because of the long times required for saturation, and that hardening temperatures are often necessary which exceed the thermal stressability, for instance, of superconducting alloys.
It is an object of the invention therefore to develop reaction resins which are useful for impregnating and/or casting large-volume parts, by which all of the foregoing requirements for a cryotechnological reaction resin molded material are met to a large degree.